Signaling by hormones and growth factors is an essential mechanism of growth regulation in multicellular organisms. Steroids are unique hormones that regulate important physiological and developmental processes in both plants and animals. Brassinosteroids (BRs), the plant steroid hormone with essential roles, share structural and functional similarities with animal steroid hormones but act through a distinct signaling mechanism. While animal steroid hormones act either through nuclear receptor transcription factors to regulate gene expression or through membrane-bound receptors to activate non-genomic responses, BRs bind to a receptor kinase at the cell surface and regulate nuclear gene expression through a signal transduction cascade. Downstream BR signaling involves both ancient signaling proteins, such as GSK3-like kinases and 14-3-3 proteins, and plant specific DNA binding proteins. My laboratory uses Arabidopsis as the model system to understand how BR regulates plant growth and development at the biochemical, cell biological, and physiological levels. We have studied how BRs regulate gene expression by controlling the phosphorylation status and activity of the key transcription factor BZR1, and we have demonstrated how BZR1 regulates the expression of BR-responsive genes. We plan to continue our study of the BR signal transduction pathway using genetic and biochemical approaches, and we plan to study how the BR signal transduction pathway is connected to various developmental processes as well as to other hormonal pathways that regulate common developmental processes. This study will advance our understanding of plant growth regulation as well as steroid signaling in general. Because steroid hormones are involved in many major diseases including cancer in human, this study will have broad implications for both agriculture and human health.